In recent years, along with the widespread use of cordless equipment such as personal computers and cellular phones, a smaller and higher-capacity secondary battery serving as a power source thereof has been much in demand. At present, as a secondary battery that has a high energy density and can be reduced in size and weight, a lithium ion secondary battery is being commercialized. The demand therefor as a portable power source is increasing. However, depending on the kinds of cordless equipment to be used, this lithium ion secondary battery has not yet been able to guarantee sufficient hours of continuous use.
Under such circumstances, as an example of a battery capable of satisfying the above-described demand, an air cell and a fuel cell can be considered. An air cell is considered to be suitable for increasing energy density because it utilizes oxygen in the air as a positive active material and allows a negative electrode to fill most of the inner volume of the cell. However, this air cell has a problem that an alkali solution used as an electrolyte solution reacts with carbon dioxide in the air and degrades, leading to a large self-discharge.
On the other hand, a fuel cell does not have such a problem. In particular, a fuel cell utilizing a liquid fuel directly for battery reaction, for example, a direct methanol fuel cell can be miniaturized and, therefore, holds a great promise as a future portable power source (see JP 2000-268836 A, for example).
Both of positive and negative electrodes of this direct methanol fuel cell contain a catalyst obtained by highly dispersing precious metal particles in carbon powder, a proton exchange resin and polytetrafluoroethylene (PTFE). The use of this PTFE as a binder makes it possible to form the electrodes having a certain strength and to provide the electrodes with water repellency (for example, see Kordesch and two others, “ECS Proceedings,” (US), 1982, vol. 82-2, No. 265, pp. 427-428).
In the above-described fuel cell, the negative electrode is supplied with the fuel and reaction occurs, while oxygen reacts in the positive electrode. Thus, by simply supplying the fuel and oxygen, the cell can be used continuously. However, since a conventional fuel cell has been constituted by layering a plurality of unit fuel cells, the total cell thickness is large. Also, it has been necessary to circulate and supply oxygen and the fuel to the negative and positive electrodes respectively, aids therefor are needed. Consequently, the fuel cell has been much larger than small-size secondary batteries such as a lithium ion secondary battery.
On the other hand, the aids for forcibly circulating oxygen and the fuel could be eliminated to miniaturize a fuel cell. However, this may cause problems that an output drops and that gases such as carbon dioxide generated in a discharge reaction build up in a fuel chamber and thus, with a consumption of the fuel, the fuel loses contact with the negative electrode.
In order to avoid the above-described problems caused by discharge products, an exhaust hole with a PTFE porous film can be provided in the fuel chamber, thereby letting out the generated gases to the outside. However, depending on the composition of the fuel, more specifically, in the case of using an alcohol aqueous solution with high concentration, there is a problem that the fuel passes through the porous film and leaks out.
Furthermore, in the case of the fuel cell in which a plurality of unit fuel cells are disposed and electrically connected, each of the unit cells has to have a sealing portion for preventing the fuel leakage. Since the fuel leakage occurs easily when the sealing is not sufficient, the structure of the sealing portion becomes more and more complicated with a view to increasing reliability. This easily causes a problem that miniaturization to a certain extent or more becomes difficult.
Moreover, not only the structure of the entire cell, but also the structure of each unit fuel cell has had room for improvement. For example, since oxygen gas reaction occurs in the positive electrode of the fuel cell, water repellency is required for removing moisture that inhibits this reaction. On the other hand, in the negative electrode where the reaction of methanol serving as a liquid fuel is to occur, the liquid fuel cannot be oxidized easily owing to poor wettability of the electrode if the electrode has water repellency. However, in the conventional direct methanol fuel cell, both of the positive electrode and the negative electrode contain PTFE as the binder and are provided with water repellency. Therefore, the negative electrode itself has not necessarily had an optimal structure.